1. Field of the Invention
The present invention relates to a laminated glass and a method of bending and shaping two overlapping glass plates simultaneously in a heating furnace to form a laminated glass.
2. Discussion of Background
A laminated glass is generally formed by laminating two glass plates and an intermediate plastic film such as polyvinyl butyral as an interlayer, and it is widely used for a windshield for an automobile from the viewpoint of safetyness.
In such laminated glass, a curved laminated glass is generally required for an automobile for requirements in designing. Accordingly, it is necessary that two flat glass plates are suitably bent to form a laminated glass. In this case, when the glass plates are separately bent, a delicate difference in shape appears between the curved glass plates to be laminated. Accordingly, when they are laminated with the intermediate film interposed therebetween, there causes disadvantages that a complete joint between the two overlapping glass plates interposing the intermediate film is not obtainable, and bubbles are resulted at the contacting surfaces during manufacturing steps or in use, or the overlapping glass plates and the intermediate film peel off. Accordingly, a method of bending simultaneously two overlapping glass plates has been employed for manufacturing a laminated glass.
As a conventional method of bending overlapping glass plates for a laminated glass, there has been utilized the method as follows. Two glass plates are put on a bending mold by interposing lubricating powder such as sodium hydrogen carbonate, cerite, magnesium oxide, silica or the like between the contacting surfaces; the bending mold on which the two overlapping glass plates are held is transferred in a heating furnace; the glass plates are heated to a processing temperature near a softening point (generally in a range of 580.degree. C.-700.degree. C. which varies depending on a kind of glass) to soften the overlapping glass plates, whereby they are bent by their deadweight along the shaping surface of the bending mold.
In the above-mentioned bending method, since a bending mold has a shaping surface with a concave form at its top, the radius of the upper glass plate after having been bent is smaller than that of the lower glass plate. Accordingly, it is necessary for the upper glass plate to be bent deeply or sharply. Therefore, it was difficult to bend sufficiently the upper glass plate in comparison with the lower glass plate, and particularly, two glass plates were not sufficiently fitted to each other at a portion where a deep-bending operation was to be applied. Accordingly, when these overlapping glass plates were laminated and bonded so as to form a laminated glass, a faulty product such as that having bubbles at the deeply bent portion might result.
In order to eliminate such insufficient bending to a side portion of a laminated glass to which a deeply bending operation is applied, it is sometimes necessary to press and shape the portion to be deeply bent by means of an auxiliary pressing member from the top of two overlapping glass plates in addition to shaping the side portion of the laminated glass by their own deadweight.
There has been proposed that a glass plate which is put on a bending mold at the upper side between two overlapping glass plates is formed to have a higher heat absorptivity than the lower glass plate so that the two overlapping glass plates are sufficiently fitted to each other at a portion where a deeply bending operation is needed (U.S. Pat. No. 3,300,351).
It is very important that the optical distortion in a windshield is substantially negligible when a curved laminated glass is used for the windshield for an automobile. Especially, where an optical distortion is a so-called lens-like distortion which has a lens-like appearance, it prohibits the eyesight of a driver, and therefore it is particularly undesirable.